A zeta potential regulator for papermaking which contains a sulfonic acid group-containing melamine-formaldehyde condensate as an active ingredient is proposed in Japanese Unexamined Patent Publication No. 259699/1985. The publication discloses that the sulfonic acid group-containing melamine-formaldehyde condensate is obtained by condensing 1 part by mol of melamine or a mixture of melamine and another condensable compound, 2 to 4 parts by mol of formaldehyde or an equivalent mols of a formaldehyde donor, and from 0.5 to 2 parts by mol of a bisulfite and a sulfite or an equivalent mols of a bisulfite or sulfite donor in a molar ratio of formaldehyde to the bisulfite or the sulfite of from 4.5:1 to 2:1.
Many sulfonated amino resins have been proposed as water reducing agents for concrete. Japanese Examined Patent Publication No. 13991/1977 proposes an aqueous solution of a sulfonic acid group-containing melamine-formaldehyde condensate obtained by reacting a melamine-urea mixture, formaldehyde and an alkali sulfite in an aqueous solution in a molar ratio of 1:2.7 to 3.2:0.9 to 1.1. Japanese Examined Patent Publication No. 43699/1989 proposes a sulfonic acid group-containing melamine-formaldehyde condensate obtained by condensing melamine, formaldehyde and a sulfite in an aqueous medium in a molar ratio of 1:2.2 to 3.2:0.3 to 1.0.
The above-mentioned sulfonic acid group-containing melamine-formaldehyde condensates are not so effective as a retention aid during sheeting, and have a drawback that it is inferior in slump retention, though they are very effective as a water reducing agent for concrete. The sulfonic acid group-containing melamine-formaldehyde condensates has another drawback that their application and amount are limited because of the remaining free formaldehyde. In addition, it is difficult to obtain a sulfonic acid group-containing melamine-formaldehyde condensate having a high viscosity by the methods disclosed in the above publications.
No report has been made about a product obtained by reacting a sulfonated amino resin and an amino group-containing substance.
The first aspect of the present invention provides a product obtained by reacting a sulfonated amino resin and an amino group-containing substance which solves the above-mentioned drawbacks of prior art and is a novel material useful in many ways, and a process for its production.
The present inventors have found that an amino group-containing substance such as melamine can be easily dissolved in an aqueous solution of a sulfonated amino resin, and accomplished the first aspect of the present invention as a result of extensive researches. Namely, the first aspect of the present invention provides a product obtained by reacting a sulfonated amino resin and an amino group-containing substance which is useful in a wide variety of industries, e.g., as a retention aid in production of paper made mainly of cellulose fibers, or cellulose fibers and inorganic filler materials, as a flocculant in waste water treatment, as a water reducing agent in preparation of hydraulic inorganic materials such as cement and gypsum, as an admixture for the earth pressure shield tunneling method, and as a dispersant for coating, its sol and a process for its production.
Paper made mainly of cellulose fibers or of cellulose fibers and inorganic filler materials is generally manufactured by supplying an aqueous papermaking stock prepared by dispersing and separating a cellulose pulp in water, adjusting the concentration and, if desired, adding filler materials and other additives, to a papermaking machine, followed by removal of water and drying processes. To improve retention of cellulose fibers and fillers, strength and quality of a paper sheet and productivity, papermaking machines, various additives such as cationic starch and papermaking processes have been improved.
It has been conventional to employ the acid papermaking process wherein aluminum sulfate is used as a flocculant and as a fixer for sizing agents, and an anionic organic polymer is used as a strength agent and as a retention aid. In recent years, inorganic filler materials are added in increasing amounts for the purposes of cost-cutting and improved quality of a paper sheet, and more low-quality pulp is used due to shortage of high quality pulp and for the purpose of cost-cutting. Further, the sheeting rate is increasing for improved productivity. The acid papermaking process, which requires much aluminum sulfate, is displaced by a modified acid papermaking process and by the neutral papermaking process in many cases because of inadequate durability of the resulting paper, insufficient reuse of white water, corrosion of facilities and unusability of calcium carbonate as an inorganic filler material. Techniques and additives developed for the conventional acid papermaking process are not sufficiently effective in the alternative processes. Therefore, retention aid and methods for improving retention are especially desired to be developed, and many proposals for the acid papermaking process and the neutral papermaking methods have been made.
It has been long known to use a cationic starch in order to cause flocculation, which is advantageous to papermaking and to improve strength of a paper sheet. Japanese Examined Patent Publication No. 6587/1980 proposes a method employing a combination of a cationic component such as a cationic starch or an acrylamide copolymer with an anionic component such as natural starch or a carboxymethylated starch, and Japanese Unexamined Patent Publication No. 12824/1980 proposes a method using a cationic resin emulsion and an anionic water-soluble polymer in combination. Japanese Unexamined Patent Publication No. 259699/1985 discloses a method wherein a zeta potential regulator containing a sulfonic acid group-containing melamine-formaldehyde condensate as the active ingredient is added before sheeting an aqueous papermaking stock containing an amphoteric polyacrylamide as a strength agent. Japanese Unexamined Patent Publication No. 14096/1990 proposes a method wherein a cationic starch is used in combination with a converted cationic starch obtained by cooking cationic starch together with from 0.2 to 15 wt %, based on the cationic starch, of an anionic polymer. As the anionic polymer, a sulfonic acid group-containing melamine-formaldehyde condensate is proposed.
The methods proposed by Japanese Unexamined Patent Publications No. 6587/1980 and No. 12824/1980 characterized by combined use of a cationic organic polymer with an anionic organic polymer, improve retention of cellulose fibers and inorganic filler materials and strength of a paper sheet, but has a low productivity due to poor drainage, and lower quality of a paper sheet such as formation.
The combination of an amphoteric polyacrylamide with a zeta potential regulator containing a sulfonic acid group-containing melamine-formaldehyde condensate as the active ingredient, as proposed in Japanese Unexamined Patent Publication No. 259699/1985, can improve strength of a paper sheet. However, in this method, retention of cellulose fibers and inorganic filler materials is poor due to weak flocculation, and the drainage is also poor. The combination of a converted cationic starch with a sulfonic acid group-containing melamine-formaldehyde condensate as an anionic polymer, as proposed in Japanese Unexamined Patent Publication No. 14096/1990, achieves good retention and drainage but can not achieve sufficient retention under certain conditions due to insufficient flocculation and has a drawback that control of the performance of a converted cationic starch is difficult.
The second aspect of the present invention provides a papermaking process which can achieve sufficient retention and good drainage in sheeting under acidic and neutral conditions by the use of a product obtained by reacting a sulfonated amino resin and an amino group-containing substance.